DC power supplies employing an inverter may be used for, for example, an arc welder, an arc cutter, a plasma arc welder and an arc lamp. These apparatuses may be designed such that they can be driven from either one of two types of commercial AC power supplies one of which supplies a voltage of a magnitude half that of the other. Voltages supplied by such power supplies may be, for example, voltages of 100 V and 200 V, or 200 V and 400 V. An example of such apparatuses is disclosed in U.S. Pat. No. 5,272,313 issued to K. Karino et al. on Dec. 21, 1993.
The apparatus disclosed by Karino et al. is an arc welder, which can be driven from either one of two AC power supplies and includes an AC-to-DC converting unit and two high-frequency inverters. The AC-to-DC converting unit rectifies and smoothes a commercial AC voltage applied to the apparatus. The voltage supplied from the commercial AC power supply currently connected to the arc welder is detected to determine whether it is, for example, 100 V or 200 V. If the voltage is 100 V, the two high-frequency inverters are connected in parallel, and a DC voltage is applied from the AC-to-DC converting unit to the parallel circuit of the high-frequency inverters. On the other hand, if the voltage being applied is 200 V, the inverters are connected in series, and the DC voltage from the AC-to-DC converter is applied to the series circuit of the inverters. High-frequency voltages from the respective high-frequency inverters are voltage-transformed in a voltage transformer, and the voltage-transformed voltages are rectified and smoothed by another AC-to-DC converting unit and applied to a load.
The arc welder of Karino et al. employs two high-frequency inverters. Such high-frequency inverters use semiconductor switching devices, such as IGBTs, bipolar transistors and FETs for high-frequency switching. As the higher switching frequency is used, the size of the voltage transformer can be smaller, which results in an apparatus having a smaller size. On the other hand, as the switching frequency becomes higher, loss (switching loss) in the switching devices when they perform switching operation becomes larger.
An object of the present invention is to provide an inverter which can operate at a higher switching frequency without causing increase of switching loss. Another object of the present invention is to provide a DC power supply apparatus which employs such inverter in order to reduce the size of the DC power supply apparatus.